Spectrum splitting systems for converting solar energy into electrical energy are known in the art. Incoming solar radiation is refracted by a refraction element (e.g., a prism) into a plurality of wavebands. Each of the refracted solar wavebands is directed onto a Photo-Voltaic (PV) cell. Each PV cell is designated such that it efficiently converts the respective solar waveband into electrical energy (i.e., the designated PV cell is designated for converting a specific waveband into electrical energy).
The refraction of the wavebands and the angle of the refracted rays depend on the approach angle between the incoming solar radiation and the refraction element (i.e., at a first angle of the sun the refracted rays are directed differently than at a second angle of the sun). Therefore, it is necessary to track the sun during its movement across the sky in order to maintain each of the solar wavebands directed onto the corresponding designated PV cell (i.e., the angle between the sun and the refraction element is maintained practically constant).
A PV solar cell converts solar radiation photons into electric charge carriers and transfers the charge carriers to opposite electrodes (i.e., electrons to an N-type electrode and holes to a P-type electrode). For the purpose of converting, the quantum efficiency and the possible thickness of the PV cell are dependant on the semiconductor material, of which the PV cell is constructed. The transfer of charge carriers depends on the life time (LT) of the charge carriers. The effectiveness of the PV cell increases with the LT of the charge carriers within the PV cell as detailed herein below.
In broadband PV cells and in tandem cells, there is an internal selection of wavebands in which different materials and structures within the cell absorb specific bands. When the cell has a stepwise varied band-gap width, it absorbs and converts the entire solar spectrum to electrical current with little energy losses to heat. Heat is the outcome of energy absorption when the energy of the impinging radiation is greater than the energy band gap of the PV cell material.
The materials employed for constructing broadband PV cells may be, for example, compounds of the group III-V (including Al, Ga, In, N, P, As and their solid solutions), or organic materials. The structures of these materials may be solid state bulk, quantum wells, quantum dots, amorphous, crystalline, or a combination thereof. The PV cell is conventionally constructed either as a PN junction or preferably a PIN junction.
Broadband PV cells can absorb and convert the entire solar spectrum to charge carriers, electrons and holes. Once they are generated, the charge carriers migrate toward the respective electrodes. That is, the probability of a charge carrier to reach its respective electrode increases with the charge carrier life-time. However, the charge carriers may encounter obstacles on their path to the respective electrodes. Such obstacles may be defects in the crystal, and natural counterpart (i.e., electron and hole) recombination, leading to annihilation. The effects of defects in the crystal are reduced by employing better substrates, cleaner source materials, well tuned processes and the like.
In broadband cells, as white light from the sun impinges on the cell, the cell becomes saturated with charge carriers having a broad range of energies, respective of the entire solar spectrum absorbed therein. A charge carrier drifts toward its respective electrode, according to an inner electric field of the PV cell. The probabilities of a charge carrier to reach its respective electrode without recombination decrease as the migration path is longer or as the migration path includes more counterpart carriers.
A material system is the coupling of a chemical element or compound with its microstructure. Each semiconductor material has a distinct energy absorption band-gap (BG), which allows it to absorb photons of that BG and higher. Microstructure corresponds to the geometry, structure, and space or lateral order regulating the atoms or molecules of the material.
An amorphous structure is a structure of relatively short order (i.e., on the order of 3-5 atoms). A nano-crystal or a quantum dot is a structure of a higher order than that of the amorphous structure (i.e., on the order of 20 atoms). A quantum well is a two dimensional sheet, the thickness of which is on the order of magnitude of a quantum dot. A poly-crystal is a structure of an order of hundreds of atoms in a row (e.g., 100 nm of ordered atoms). A multi-crystal is a poly-crystal of several millimeters long. It is noted that, a single crystal (i.e., mono-crystal of a few millimeters long can be cut from a multi-crystal.
Silicon, an abundant PV cell element, appears as a nano-crystal, a mono-crystal, a multi-crystal, a poly-crystal, or as an amorphous material. A single PV cell can include a combination of the above microstructures for increasing the conversion efficiency of the cell. For example, the conversion zone of a PV cell, containing nano-crystal (nc-Si) silicon grains and amorphous silicon (a-Si), is broader than that of a PV cell containing only a single microstructure. A PV cell of a-Si converts photons with energy of 1.7 eV and higher. A PV cell of combined microstructures of a-Si and nc-Si converts photons having energy of 1.15 eV and higher. Compound semiconductor systems may convert photons of higher energies (i.e., broader wavebands). A PV cell with a combination of nano-crystalline (i.e., Quantum Dots) InAs with quantum wells of GaAs, spaced by layers of AlGaAs may convert the entire solar spectrum.
A PV cell may be constructed such that each refracted solar waveband is directed to a corresponding material having similar band gap. For example, three wavebands of 200-450 nm, 450-650 nm, and 650-1000 nm are captured by three corresponding conversion zones, AlGaP having BG of 2.7 eV, GaAsP having BG of 1.85 eV, and InGaAs having BG of 1.2 eV, respectively.
One method known in the art for fitting photon waveband to material BG is horizontally splitting white light, coming from the sun, into several wavebands (e.g, blue, green, red and infra-red), and to convert each waveband with a material of corresponding BG. Another method is to apply vertical internal selection of colors. Layers of converting materials are stacked vertically, ordered according to their BG. For example, the top layer (i.e., closest to the sun) converts blue waveband, and the bottom layer (i.e., farthest away from the sun) converts IR waveband. The blue converting material layer absorbs only photons with energies higher than blue color, and allows photons with lower energies to pass through, and be absorbed in the layers underneath.
There are two main approaches known in the art for utilizing the second method described above. The first approach is the multi-junction approach (i.e., tandem cell). For example, three sub-cells or PN junctions are stacked over each other ordered according to their BG. The three sub-cells are serially connected by tunneling junctions. The current produced at individual cells passes throughout the stack and is collected at an electrode positioned at the end of the stack. The current collected by this approach is limited to the lowest current of all the sub-cells. In order to keep the currents of each of the sub-cells equal, very accurate BGs must be engineered, and the thicknesses of each sub-cell must be precise.
The second approach is to provide either a single PN or a single PIN cell in which the BG is stepwise varied, decreasing from the light receiving front. The structure of the second approach is formed by inserting quantum wells of varying thickness to a host compound semiconductor material. The quantum wells include quantum dots of a lower BG material, and are of predetermined sizes. Since the BG of each of the quantum wells is at inverse proportion to its thickness, the BG of the PV cell is stepwise varied, allowing conversion of the solar spectrum.
U.S. Pat. No. 6,015,950, issued to Converse, and entitled “Refractive Spectrum Splitting Photovoltaic Concentrator System” is directed to a solar energy conversion system. The system includes two pluralities of refracting elements (i.e., prisms) and two types of solar energy converters (i.e., a first energy converter designed to convert a first band of wavelengths, and a second energy converter designed to convert a second band of wavelengths). Each of the pluralities of refracting elements disperses oncoming broad spectrum light and redirects a portion of the spectrum of the oncoming light onto a different type of solar energy converter. Each of the solar energy converters converts the redirected portion of the spectrum, of the oncoming light, into electrical energy. The surface area of the converters is smaller than that of the pluralities of refracting elements, such that the oncoming light is focused onto the solar energy converters. A mounting arrangement holds the prism arrays and the photovoltaic cells fixed with respect to each other. The mounting arrangement tracks the sun so that the prism arrays are preferably held normal to the incident sunlight.
U.S. Pat. No. 7,206,142, issued to Wagner, and entitled “Refractive Spectrum Splitting Concentrator System” is directed to a system for concentrating and refracting electromagnetic energy having a broad energy spectrum onto bands of a target device. The system includes a Fresnel lens and a target device. The Fresnel lens refracts the electromagnetic energy. The Fresnel lens further concentrates specific wavelengths onto rectilinear bands on the target device. The target device is a solar cell. Each of the rectilinear bands on the target device corresponds to a different range of wavelengths.
US patent Application No. 2002/0003201, to Yu, entitled “Image Sensors Made From Organic Semiconductors” is directed to a multi-color image sensor made from organic semiconductors. The image sensor includes a prism or a micro-prism array, and three identical broad band photo-sensors. The prism is located in front of the color sensors and refracts incoming light into three colors (e.g., red, blue and green, although any other number and combination of colors is possible). Each of the identical broad-band photo sensors is a multi-layer structure of organic semi-conducting materials. Each of the three identical photo-sensors senses each of the refracted colors.
U.S. Pat. No. 6,566,595 to Suzuki, entitled “Solar Cell and Process of Manufacturing the Same”, is directed to a solar cell employing a quantum dot layer in a P-I-N junction. The solar cell includes a p-type semiconductor layer and an n-type semiconductor layer made of a first compound semiconductor material. At least one quantum dot layer is formed between the P-type semiconductor layer and the N-type semiconductor layer. The quantum dot layer is constructed of a second compound semiconductor material and has a plurality of projections (i.e., quantum dots) on its surface. The quantum dots are of different sizes on a single quantum dot layer, or on any one of the quantum dot layers.
The quantum dot layer is inserted in the I-type semiconductor layer of the P-I-N junction. Thus, light of wavelength corresponding to the practical forbidden band width of the quantum dot layer is absorbed, in addition to light of wavelength corresponding to the forbidden band width of the semiconductor material forming the P-N junction. This increases the photoelectric conversion efficiency of the solar cell. The forbidden band width of the quantum dot layer can be varied depending on the combination or compound crystal ratio of the semiconductor used for forming the quantum dot layer. Thus, the wavelength range in which the photoelectric conversion can be carried out may be extended, and a solar cell which allows photoelectric conversion of varying wavelengths at high efficiency corresponding to the incident light can be manufactured. In a process of manufacturing the solar cell according to Suzuki, the quantum dot layer may be formed by lithography and selective etching, or by self-growing mechanism. The semiconductor material used for forming the quantum dot layer may be a compound of a group III element and a group V element shown in the periodic table, such as InGaAs or GaAs.
US Patent Application Pub. No. US2005/0155641 to Fafard, entitled “Solar Cell with Epitaxially Grown Quantum Dot Material”, is directed to a photovoltaic solar cell having a sub-cell structure, and to a method for making such a solar cell. The solar cell is a monolithic semiconductor photovoltaic solar cells including at least one sub-cell, having a self-assembled quantum dot material. Each of the sub-cells of the solar cell exhibits a different bandgap energy value, and thus absorbs photons of different wavelengths. The sub-cells are disposed in order of increasing effective band gap energy, with the sub-cell having the lowest effective band gap energy being closest to the substrate. A barrier semiconductor layer is formed between each pair of sub-cells of the solar cell.
The method for making the solar cell includes epitaxial growth of the quantum dot material. The growth temperature of the quantum dot layers is used to adjust the shape and composition of the quantum dots. The temperature during the overgrowth of the barrier of each quantum dot layer may be varied at different stages of the overgrowth, to further control the size and composition of the quantum dots and therefore the absorption characteristics of self-assembled quantum dot material. The combination of epitaxial growth parameters is chosen to obtain quantum dot layers having a high in-plane density of highly uniform quantum dots having desired energy levels. Such growth parameters are: growth temperature, the group-V over-pressure or the III/V ratio, the quantum dot material, the amount of material used to obtain the self-assembled growth transition between a uniform quasi two-dimensional film to three-dimensional islands, the growth rate or the pauses used during the growth, and the overgrowth conditions such as growth temperature and growth rate.
A review “Spectral Beam Splitting Technology for Increased Conversion Efficiency in Solar Concentrating Systems”, to A. G. Imenes and D. R. Mills, Solar Energy Materials & Solar Cells 84 (2004) 16-69, is directed at solar beam splitting systems proposed in the literature and different spectrum splitting strategies. In particular, section 4.2 of Imenes is directed to refractive and absorptive filtering methods. Imenes discloses a system including a prism, and a plurality of single-band gap cells ordered in an increasing band-gap order. The prism disperses white light and directs each of a plurality of single band light rays onto a respective cell.